Generally, an amount of communication traffic rapidly increases in a natural disaster or artificial disaster, such that a user, who meets with misfortune, has difficulty in connecting his or her urgent call such as an emergency rescue call. Specifically, in order to support a communication mode for emergency rescue in the next generation communication system, E-911 and requirements are being reflected in this next generation communication system. The above-mentioned requirements enable resources to be properly maintained in a range from the beginning of a communication mode to its maintenance status, and implement a priority-based communication processing, thereby supporting high-priority communication such as an emergency rescue call. In order to support the emergency rescue communication, a variety of methods should be applied to a communication system, for example, allocation of priority to an emergency call, reservation of radio resources, and a preemptive access to a call path.
Generally, at the beginning of communication, system information is obtained from a downlink channel, a UE access is carried out according to predetermined system parameters, and a system allows registration/access of a call control according to a UE performance, such that a communication mode can be maintained. A user multiplexing is required for a cellular network, such that the UE is able to the initial uplink without conducting uplink synchronization in only a specific time interval. This channel is called a random access channel, the UE generally transmits signals to this random access channel, and a base station (BS) estimates an uplink timing error, such that a timing advance can be provided to the UE. This random access channel has been generally designed irrespective of a UE type or call priority. So, the UE access is abruptly carried out if any emergency situation occurs, resulting in the occurrence of congestion. Therefore, under the condition that the base station (BS) is unable to recognize which one of UEs has higher priority, the base station (BS) has to conduct the call control. As a result, the base station (BS) must assign priority to only a UE of a successful initial access in such a way that communication between the base station (BS) and the UE is conducted.
The random access channel of a conventional system is adapted to reserve logical or physical resources of the system, and the UE transmits a specific sequence via the reserved resources. In this case, the above-mentioned UE access can be classified into a UE pre-access in which the UE has previously accessed the system and a UE initial access.
If the UE has previously accessed the system, it updates conventional timing advance information over the random access channel, and transmits information of a channel status. A corresponding UE transmits a corresponding sequence at a specific time (e.g., a time decided by the UE and a time decided by a base station).
In case of the initial access, the UE directly accesses the random access channel without any limitation or indication, and selects a predetermined sequence to be used for the corresponding random access channel. In this case, if the UE detects a sequence transferred from the UE, it transmits additional operation information to a downlink. If the UE monitoring a downlink channel receives information corresponding to the sequence which has been transferred from the UE itself, it gains access to the system according to the corresponding information. Otherwise, if there is no response during a predetermined time, the UE performs a backoff during a predetermined time, and modifies a combination of power/random access channel/sequence, such that it re-attempts to access the random access channel. The above-mentioned process can also be carried out in a handoff process.
The above-mentioned process has no problems under small amount of load. However, provided that the system is overloaded, the number of reattempts of UEs increases, such that a ranging channel must unavoidably undergo congestion. That is, the number of collisions between UEs increases, and an interference level between signals also increases. Therefore, a system access time of the UE may be delayed, such that it is contrary to the order of call setup steps in an emergency situation.
Therefore, conventionally, a random access channel under an emergency situation is designed to be operated in the same manner as in ordinary cases, such that it is unable to support communication based on priority (i.e., priority-based communication).